With increased gas firing temperatures in modern day gas turbines, various portions along the hot gas path of the turbine are exposed to a higher heat load environment. One area that has exhibited marked increase in heat load is the tip of the airfoil on the gas turbine rotor. Excessive heat loads have resulted in airfoil tip cap oxidation and creep distress. While thicker and different materials less subject to oxidation and creep properties have been substituted for prior airfoil tip cap designs, it has been demonstrated that unacceptable tip cap stress levels may increase notwithstanding these thicker and different materials. Accordingly, there is a demonstrated need for a cooling system for the airfoil tip cap of a turbine bucket which will lower the tip cap metal temperature and thermally-induced stresses and particularly without increasing the total turbine cooling flow.
In a preferred embodiment of the present invention, there is provided a bucket for a gas turbine having an airfoil, a shank and a platform between the shank and the airfoil. The airfoil includes a cooling circuit having a plurality of passages for flowing a cooling medium within the airfoil and at least one rib extending between opposite sides of the airfoil dividing the cooling circuit into a forward cooling circuit and an aft cooling circuit. The airfoil also includes an airfoil tip having an opening and a tip cap secured to the airfoil closing the opening. The rib includes a hole for flowing a portion of the cooling medium toward the tip cap to impingement cool the tip cap.
In a further preferred embodiment of the invention, there is provided a bucket for a gas turbine having an airfoil, a shank and a platform between the shank and the airfoil, the airfoil including a cooling circuit having a plurality of passages for flowing a cooling medium within the airfoil. The airfoil includes a tip having an opening and a tip cap secured to the airfoil closing the opening. The tip cap has a thermal barrier coating along an outside surface thereof. Preferably, the airfoil has a seat adjacent the airfoil tip for receiving the tip cap and which seat also has a thermal barrier coating.